1. Field of the Invention
The invention relates to die attach pickup tools for picking up a flexible die attach from a cutting station, transferring the die attach to a bond site and depositing the die attach on the bond site, more specifically for use in relocating flexible adhesive pads for bonding integrated circuit chips to substrates.
2. Description of the Prior Art
In the mass production of integrated circuit (IC) chip assemblies, a particularly critical step to the process is the step of bonding the IC chip to its substrate. Such substrates include lead frames, MLC chips, diodes, quartz crystals and the like.
Heretofore, IC chips have been bonded to the substrate with relatively inflexible adhesive pads of eutectic alloy or soft solder die attach ribbons. Mechanical devices or die bonders, as they are known in the art, have employed tools to pick up the relatively inflexible pads from a cutting station, to transfer them to a heated bonding site on a substrate, and to deposit them on the bond site of the substrate with slight pressure to effect bonding of the pads to the substrate. The heat of the site melts the pads, typically causing the pads to deform and flow as a liquid with concomitant reduction of their surface area.
Tools used in the art for this procedure consist of a hollow tubular element 10, such as illustrated in FIG. 1. One end 2 of the element connects to a vacuum source. Another end 3 of the tool contacts the inflexible adhesive pad or die attach. In operation, the vacuum source is turned on when the tool is programmed to pick up the die attach, transfer it, and deposit it on the bond site. The vacuum is turned off when the tool is programmed to release the die attach and return to the cutting station.
Subsequently, another mechanism sets an IC chip on the adhesive pad.
There are clear benefits in using a relative flexible thermoplastic polymer as the adhesive pad. A bonding process using such flexible thermoplastic pads is disclosed in copending U.S. patent application Ser. No. 088,141.
However, conventional pickup tools cannot be used to relocate flexible adhesive pads. Typically, the hole 4 in the end 3 of the tool is too large causing the flexible pad to be partially sucked into the tool hole 4 when the vacuum source is on. Faces of the tools which contact inflexible adhesive pads are smaller than the relative size of the pads being relocated. This causes a similarly sized flexible die attach to sag away from the prior art tool when picked up. These problems have further repercussions when the tool approaches a heated bonding site which softens the flexible pad. This softening further allows the pad to be sucked into the tool and curls the pad. When these deformations occur, the pad does not deposit flat, but in a partially folded form. The pad also tends to form gas pockets due to the non-flat placement. Further, the pressure required to transfer the pad from the conventional tool to the bond site typically leaves an impression on or cuts the flexible pad. At least in part due to these problems, conventional tools do not result in flexible pads being relocated in a uniform and reproducible manner.
From the foregoing description of the current state of the art, it is evident that there is a very substantial unmet need for pickup tools capable of relocating flexible die attach pads such that the pads are maintained in a substantially flat shape throughout the process.